The present invention relates generally to optical communications, and, more particularly, to a colorless differential phase shift keying demodulator for suppressing inter-symbol interference in dense wave division multiplexing communication systems.
Internet-based traffic has been growing exponentially due to the rapid increase of the microelectronic processing power, expansion of communication networks towards ubiquity, and emerging of modern bandwidth-thirsty business and personal applications such as video-on-demand (VoD) and storage area network (SAN). As the backbone to provide the transportation pipelines for such traffic volumes, the optical network has received demands for larger bandwidth capacity.
As a result, the 10 Gb/s bandwidth per channel in dense wavelength division multiplexing (DWDM) optical system is becoming inadequate. DWDM system with higher transmission rate of 40 Gb/s has begun to be deployed in long haul and metro optical networks.
In these new DWDM systems, the conventional on-off-keying (OOK)-based intensity modulation scheme, also called non-return-to-zero (NRZ), has very limited performance in long-haul optical transmission. New modulation formats such as differential phase-shift-keying (DPSK), differential quadrature phase-shift-keying (DQPSK) and duobinary have been developed to mitigate the fiber detrimental effects, achieve higher ONSR tolerance and/or deliver better spectral efficiency.
Among these new modulation formats, optical DPSK has become a popular candidate for 40 Gb/s DWDM transmission due to its tolerance to fiber nonlinearities and higher receiver sensitivity. It offers 3 dB OSNR improvement with a balanced receiver and a decrease of self-phase modulation (SPM) and cross-phase modulation (XPM) due to the constant envelope modulation. Although DPSK has superior transmission performance, its relatively broad spectrum limits the spectral efficiency of DPSK-based DWDM systems. Under 50 GHz ITU grids, 40G DPSK signal suffers from inter-symbol interference (ISI), where optical pulse width broadening due to narrow filtering leads to the interference between neighboring bits. The optical filtering effect in 50 GHz spaced systems leads to interference between neighboring signal bits. This phenomenon is known as the inter-symbol interference (ISI). The ISI effect can cause a dramatic increase in signal bit error rate.
FIG. 1 shows the schematic of a typical DWDM transmission system with 40 Gb/s DPSK signal on each DWDM channel. At the input side, each DWDM channel has a respective DPSK transmitter 1011 to 1014, which uses a differential encoder 1011 to apply one-bit-delay exclusive OR operation on the 40 Gb/s data, and modulates the phase of the continuous wave (CW) laser light 1012 at the phase modulator 1013. Each laser source has its respective DWDM wavelength on ITU-T grid. The output is an NRZ-DPSK signal. This signal is further intensity modulated 1015 with a driving clock 1014 signal to carve the pulse and reduce the phase chirp. So the final output of the transmitter is an RZ-DPSK or CSRZ (carrier-suppressed RZ)-DPSK signal. These 100 GHz spaced DPSK channels are combined using a 100 GHz AWG-based multiplexer 1031. Another set of multiplexer combined 1032, 100 GHz spaced DPSK channels (with 50 GHz center frequency offset to the first set) is combined with the first set using an optical 100 GHz to 50 GHz interleaver 107. After traveling through the optical link 1151, 1152 with intermediate repeaters 1051 to 1053, the DWDM signals are separated at the receiving node by 50 GHz to 100 GHz de-interleaver 109 and 100 GHz demultiplexers 1111 1112. The demultiplexed individual channels are then sent to DPSK receiver 1131-1134, which contains a delay interferometer (DI) 1131 and a pair of balanced detectors 11321, 11322. The DI uses the interference between the preceding bit and current bit to convert the phase modulated signal into an intensity modulated signal. The balanced detector can use the two output ports from the DI (the constructive port and destructive port) and improve the sensitivity of the receiver.
This schematic of FIG. 1 shows that each DWDM signal travels through several passive optical filter elements between the transmitter and the receiver. These elements include a multiplexer and demultiplexer, interleaver and de-interleaver. These optical elements cause a strong optical filtering effect to the 40 Gb/s DPSK signals, which broadens the 40 Gb/s optical signals and results in the extension of signal energy into the time slots of neighboring bits. The narrower the passband profile of these optical filter elements, the stronger the filtering effects on the 40 Gb/s signal. Other factors such as passband shape (flat-top or Gaussian), passband asymmetry, insertion loss ripple and center frequency offset will also affect the level of the filtering effect. There might be other filtering elements in the transmission link, such as wavelength blocker in optical add/drop multiplexer nodes. The eye diagrams of FIGS. 2A and 2B show the 33% RZ-DPSK signal at receiver before and after the 100 GHz AWG multiplexer and 100 GHz to 50 GHz interleaver. The ISI effect caused by these filtering elements is clearly demonstrated.
Proposed methods to mitigate ISI effect or to reduce the ISI problem caused by the strong filtering effect include use of spectral efficient modulation schemes; coding; side band pre-filtering methods; electronic equalization and optical equalization.
Use of spectral efficient modulation schemes such as optical duobinary and DQPSK modulation schemes can achieve 33 GHz at 90% spectral width. Therefore the signals are more tolerable to the filtering effect caused by the optical elements. However the duobinary signal has poor tolerance to nonlinear effect and therefore cannot has limited transmission span. The DQPSK modulation requires more complex and expensive transmitters and receivers.
An advanced coding scheme can be used to introduce correlation of the signal and control the power spectral density, and even lead to a reduction of signal spectral width. The downside is that the implementations are still technically challenging or very expensive for applications at a high speed such as 40 Gb/s.
Side band pre-filtering methods such as single-side-band (SSB) filtering and vestigial-side-band (VSB) filtering reduce the optical signal spectral width (to as much as half) to better fit into the passband width of the optical channel. The disadvantage is the increased complexity and compromised signal performance.
Electronic equalization such as electronic post-detection processing is used to improve system performance. The operation is typically based on feed-forward equalizers (FFE), decision feedback equalizers (DFE), maximum likelihood sequence estimation (MLSE), etc. It is shown that electronic equalization can partially cancel ISI and lead to an opening of the receiving signal eye. However, the performance of EDC is limited because the phase information of the incoming optical signals is lost due to OE conversion. Optical equalizers can be applied together with EDC.
An optical equalizer technology developed by applicants previously, an intra-channel optical equalizer, is a special optical filter. It is known that for a signal pulse not to have ISI it must satisfy the Nyquist criteria, and some popular Nyquist pulses have raised-cosine profile for their Fourier transforms. Therefore, it is desirable to set the transfer function of the band limited channel to a raised-cosine shape. Based on the given profiles of the passive optical filtering elements in the optical link, a corresponding optical equalizer is designed to complement them and produce an overall raised-cosine profile as shown in FIG. 3A for a multiplexer element, FIG. 3B for an interleaver element, FIG. 3C for combined filtering effect from multiplexer element and interleaver element, FIG. 3D for an optical equalizer element and FIG. 3E for an overall filtering effect with optical equalization.
The filter has a periodic profile with free spectral range (FSR) equal to the channel spacing of the DWDM signal and center frequency locked to the ITU-T channel grid, therefore it works on all the DWDM channels within the band.
Simulation results show about a 6 dB Q factor improvement for the back-to-back signals and 3 dB improvement after transmission over about 500 km fiber.
An optical equalizer with such a scheme can be designed based on Fabry-Perot (FP) interferometer theory and fabricated using dielectric thin-film technology. Comparison of ISI suppression without optical equalization, see eye diagrams 4A, 4B, and with optical equalization, see eye diagrams 4C, 4D, for a 40 Gb/s DPSK signal shows an improvement in the receiving signal particularly for the constructive port FIG. 4C.
A disadvantage of an optical equalizer is the requirement of an additional optical element in the transmission link. Also, as an athermal device without a temperature control mechanism, it might have temperature drift and have center frequency offset to the DPSK demodulator.
Accordingly, there is a need for an optical solution that integrates the functions of a DPSK demodulator and optical equalizer to reduce the inter-symbol interference ISI from the filtering effect on the optical path.